The possibility of identifying user geographical location in the network has enabled a wide variety of commercial and non-commercial services, including navigation assistance, social networking, location-aware advertising, emergency calls, etc. Different services may have different positioning accuracy requirements imposed by the application. In addition, certain regulatory requirements on the positioning accuracy for basic emergency services exist in some countries, i.e., FCC requirements for E911 calls in the United States.
In many environments, the position can be accurately estimated by using positioning methods based on GPS (Global Positioning System). Nowadays networks also often have the possibility to assist user equipment (“UEs”), to improve their receiver sensitivities and GPS startup performance. Such assistance procedures are referred to as Assisted-GPS positioning, or A-GPS. GPS or A-GPS receivers, however, may not necessarily be available in all wireless terminals. Furthermore, GPS-based positioning often fails in indoor environments and urban canyons because of poor GPS reception in such circumstances. A complementary terrestrial positioning method, called Observed Time Difference of Arrival (OTDOA), has therefore been standardized by the Third Generation Partnership Project (3GPP).
In accordance with OTDOA-based positioning, a UE measures the timing differences for downlink reference signals received from multiple distinct locations, e.g., from multiple cells in a wireless communication network. For each (measured) neighbor cell, the UE measures a Reference Signal Time Difference (RSTD), which is the relative timing difference between a neighbor cell and a reference cell. The UE position estimate is then found as the intersection of hyperbolas corresponding to the measured RSTDs. At least three measurements from geographically dispersed base stations with a good geometry are needed to solve for two coordinates of the terminal and the receiver clock bias.
Precise knowledge of the transmitter locations and transmit timing offset are needed to solve for the UE's position. Position calculation can be conducted, for example, by a positioning server in the network, e.g., an E-SMLC in a Long Term Evolution (LTE) network, or the UE may carry out at least a portion of the calculations. The former approach corresponds to the UE-assisted positioning mode, whilst the latter corresponds to the UE-based positioning mode.
To enable positioning in LTE and facilitate positioning measurements of a proper quality and for a sufficient number of distinct locations, new physical signals dedicated for positioning have been introduced and low-interference positioning sub-frames have been specified in 3GPP. The new physical signals dedicated for positioning measurement are referred to as “positioning reference signals” or PRSs.
Conventionally, for a given cellular network transmitter, PRSs are transmitted from one antenna port (R6) according to a pre-defined pattern. For example details on PRS patterns, see section 1.1.2 of the Technical Specification identified as 3GPP TS 36.211, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation”, v9.1.0, 2010-03-30 (hereafter “TS 36.211”). As a particular approach to patterning, a frequency shift that is a function of Physical Cell Identity (PCI) can be applied to the specified PRS patterns, to generate orthogonal patterns modeling an effective frequency reuse of six. Doing so makes it possible to significantly reduce neighbor cell interference on the measured PRSs and thus improve positioning measurements.
Even though PRSs have been specifically designed for positioning measurements and in general are characterized by better signal quality than other reference signals, the standard does not mandate using PRS. Other reference signals, e.g. cell-specific reference signals (CRSs), therefore, at least in principle, can be used for positioning measurements. However, as noted, PRSs are dedicated for use in making positioning measurements.
PRSs are transmitted in pre-defined positioning sub-frames grouped by several consecutive sub-frames NPRS, which constitute one positioning occasion. Positioning occasions occur periodically with a certain periodicity of N sub-frames. That is, the time interval between two positioning occasions defines the periodicity of positioning occasions, specified in the number of transmission sub-frames. The standardized periods N are 160, 320, 640, and 1280 ms, and the number of consecutive sub-frames are 1, 2, 4, and 6.
Further, as previously noted, Section 1.1.1 of TS 36.211 provides that PRSs are transmitted within a given LTE network cell from one antenna port (R6) in accordance with a pre-defined pattern. The currently specified PRS patterns may be understood in terms of the resource elements (REs) within a block of 12 subcarriers over 14 OFDM symbols (1 ms sub-frame with normal cyclic prefix).
A set of frequency shifts can be applied to the predefined PRS patterns to obtain a set of orthogonal patterns which can be used in neighbor cells to reduce interference on the PRSs and thus improve positioning measurements. That is, these techniques, which model an effective frequency reuse of six, reduce PRS interference between cells and thereby improve the quality of PRS measurements made by any given UE with respect to the PRSs being transmitted for a given cell. As for the frequency-shift based reuse, the frequency shift is defined as a function of Physical Cell ID (PCI) as follows:vshift=mod(PCI,6).Notably, PRSs from any given cell can be muted for all or part of a given positioning occasion. Such muting constitutes transmitting the PRSs at zero or substantially-reduced power (as compared to non-muted transmission powers). See FIG. 2 for an example illustration of frequency shifting and see 3GPP TS 36.355, “Evolved Universal Terrestrial Radio Access (E-UTRA); LTE Positioning Protocol (LPP)”, v9.0.0, 2010-01-05 (further referred to as “TS 36.355”), for more muting details.
To improve detection of PRS, i.e. to allow for detecting PRS from multiple sites and at a reasonable quality, positioning sub-frames have been designed as low-interference sub-frames (LIS) and data transmissions are suppressed in general in positioning sub-frames. This approach means PDSCH shall not be transmitted to the UE during the PRS sub-frames. The result is that in synchronous networks, PRSs are ideally interfered with only by PRSs from other cells having the same PRS pattern index (i.e. same vertical shift, vshift) and not by the data transmissions.
In addition to the use of LIS, PRSs can also be transmitted during the sub-frames configurable for the Mobile Broadcast Single Frequency Network (MBSFN). These sub-frames do not contain user data, and only the first 2 OFDM symbols in each MBSFN sub-frame may contain common control channels (e.g., PDCCH) or physical signals (e.g., CRS). In LTE, up to 6 downlink sub-frames in a frame can be configured for the MBSFN. Due to no data transmissions, the interference is reduced in these sub-frames.
In the case of both LIS and MBSFN sub-frames, an important observation is that there exist REs whereby there are: (a) resource elements exclusively for PRS (REPRS); (b) resource elements with non PRS signals (RENON-PRS); and (c) resource elements which are completely empty or unused (REUNUSED). Note that the unused REs, REUNUSED, may be considered as a subset of the non-PRS REs, RENON-PRS.
In partially aligned asynchronous networks, PRSs can still be interfered with by transmissions over data channels, control channels or any physical signals when positioning sub-frames collide with normal sub-frames. This effect is minimized by partial alignment, i.e. by aligning the beginning of positioning sub-frames in multiple cells within one-half of a sub-frame with respect to some time base. Furthermore, in practice, interference on PRS REs may also be caused by other factors such as poor synchronization or large delay spread.
In any case, in using PRSs for OTDOA positioning, a UE may have to deal with PRSs from a neighboring cell that are much weaker than those received at the UE from its own (serving) cell. Furthermore, without the approximate knowledge of when the PRSs are expected to arrive in time and according to what pattern, the UE is obligated to perform signal searching within a large time and/or frequency window, which negatively impacts the time and accuracy of PRS measurements and increases UE complexity.
To facilitate PRS measurements by UEs, the network transmits assistance data, which includes, among other things, reference cell information, a neighbor cell list containing the PCIs of neighbor cells, the number of consecutive downlink sub-frames, PRS transmission bandwidth, frequency, etc. Of course, as noted, the PRSs in any given cell may be muted at given positioning occasions. If a given cell mutes its PRS transmissions for given positioning occasions, that muting generally applies for all PRS REs within the same sub-frame over the entire PRS transmission bandwidth.
On this point, certain optional signaling regarding muting is specified in TS 36.355, but there is no standardized muting pattern. Further, it remains open whether such signaling will be applicable to asynchronous networks. Still further, the possibilities for coordinated muting among transmitting nodes are limited. Thus, the contemplated muting signaling may not be very helpful, for example, for home eNodeBs.
Additionally, the use of blank subframes for “hetnets” (heterogeneous networks) and related signaling is not defined, and there is the possibility that PRS muting pattern is masked by the general hetnet pattern. As a further complication, it is worth noting that hetnet-related muting or puncturing can be applied not only over certain subframes, but also in frequency over certain subframes. Further, muting has been specified as a non-critical extension for Rel-9, meaning that there may be Rel-9 UEs that do not support muting signaling.
In principle, PRS muting can also be applied on OFDM symbol basis. This implies that the PRS muting need not be applied in all of the OFDM symbols in the sub-frame. However, all PRS REs within an OFDM symbol are muted. According to the current formulation presented in 3GPP TS 36.213, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures”, v9.1.0, 2010-03-30, (further referred to as “TS 36.213”), the PRS muting shall be applied only on a positioning occasion basis (i.e., it is proposed that the transmit power on PRS will be constant over the positioning occasion). However, as noted, the hetnet pattern may not follow the PRS pattern, and, in general, muting may be applied on a sub-frame basis, or even decided at the symbol level. Furthermore, positioning measurements are not required to be performed on PRS, they may be performed, for example, on CRS or other signals for which no muting information may be signaled at all.
A muted PRS or punctured RE is similar to a non-scheduled RE or otherwise “empty” RE. Because of imperfections in signal transmission, reception, and processing, and because of interference and other impairments, even a non-scheduled PRS RE transmits noise. However, even allowing for interference noise power, the transmit power level of a PRS RE when PRS muting is applied is typically well below the transmit power level of a PRS RE when PRS muting is not applied. For instance, the difference between the un-muted and muted PRS levels can be in the order of 30 dB (especially for hetnet).
Muting or puncturing may also be random, where each cell (eNodeB) selectively mutes its PRS transmissions, and where the decision to mute or not is made with some probability. In a simple implementation of random muting, neighboring eNodeBs do not coordinate their muting decisions. The probability at which any given eNodeB makes its selective muting decisions may be statically configured. One advantage of this approach is that no coordination signaling is needed among eNodeBs, as the muting decisions are made locally for each eNodeB or cell. One disadvantage is that real networks are inhomogeneous, with different cell coverage areas and user densities, and possibly different types of base stations. The optimal configuration of such probabilities may also vary, for example, over the day and over the week and on the cell basis, which makes static configurations not the best option from the practical point of view.
These variations imply that setting optimal muting probabilities is a tedious task. Furthermore, there may be multiple patterns in the network (e.g. PRS pattern and hetnet pattern). Even if optimal muting probabilities could be set or maintained for the eNodeBs, the UEs operating in the corresponding cells of the network would not be appraised of the muting decisions. As such, a given UE would not know whether the PRS from a given cell is or is not muted for any given positioning occasion. Without such knowledge, the UE does not know whether to make PRS measurements with respect to that cell, which adds to UE complexity and can degrade PRS measurement results.
One approach that addresses at least some of the issues, including random muting, involves designing a limited set of muting patterns and mapping the muting pattern IDs to PCIs. This approach offers the advantage of providing a table of muting patterns and PCI in the assistance information, meaning that a UE can use the assistance information to determine when the PRSs in a given cell of interest are muted. As a disadvantage of this approach, however, the muting patterns either need to be hard coded in the UEs or received from the network.
The first solution is not appropriate for all UEs, e.g., older UEs and any Rel-9 UEs that do not implement support for muting signaling, while the second solution adds to the signaling requirements of the network. In any case, mapping muting patterns to PCIs will most likely not result in an optimal muting configuration in non-uniform real networks that may also have a multi-layer structure. In other words, with this approach, the muting configuration is fixed and is impossible to re-optimize unless PCI planning is redesigned for the entire network specifically to complement positioning, which likely is not a top priority from the network operator's perspective.
One state-of-the-art approach to blind detection uses correlation principles to detect the presence or absence of a known reference or pilot signal. According to this process, a UE correlates the received signal with all possible pre-defined reference signal waveforms and compares the correlation result to an absolute threshold. No selective measurement over subframes or symbols or REs is performed, to exclude certain PRS REs within the total measured interval. Low correlation indicates that the signal is absent, i.e., not transmitted within the considered search window.
If the comparison is made based on (coherent and/or non-coherent) accumulations over all measured sub-frames over the entire measured interval, then muting detection is essentially the same as the classical signal detection. When the muting pattern is not known to the UE and the measured signal can be muted in shorter intervals, i.e. the muting may occur in some sub-frames and may not occur in the other sub-frames within the same measured interval, then with the state of art, the signal would still be accumulated over the entire measurement interval even though in some sub-frames the signal is not present. With a known muting pattern or a known set of punctured REs, the non-present PRS REs may be excluded from the correlation, but without this knowledge and with a full muting flexibility (e.g., when muting can be applied in any sub-frame or its part, any part of the bandwidth in the muted sub-frame and during any time interval) the UE would be obligated to implement blind detection of muting for any such sub-frame, bandwidth or their parts.
This implies that the advantage of signal accumulation, which increases the correct detection probability and is necessary at low Signal-to-Interference Ratios (SINRs) when the detection probability is low, cannot be exploited if flexible muting is in use by the network—unless, of course, the UEs are appraised of the muting patterns in use. This problem is exacerbated in the sense that a given UE must reliably detect and measure PRSs from multiple cells (e.g., even more than the theoretically-required three cells).
Thus, while signal accumulation improves correlation detection performance, it is, in practice, difficult for the UE to maintain an appropriate absolute threshold for evaluating the correlation results. Hence, non-muted portions of the PRSs or more specifically the resource elements in which the PRS is not muted may be mistakenly identified or detected or estimated by the UE or the target device as being muted, or vice versa. These mistakes or detection errors degrade the PRS measurements and, correspondingly, the accuracy of positioning determinations.